This invention relates generally to injection molding machines, and more particularly, to injection molding machines including means for controlling flow of molten material into a mold cavity of a machine molding assembly designed for molding a centrally apertured record disc, such as a video information disc, and for molding a central aperture into such disc.
An example of an injection molding machine of this general type is disclosed in copending and commonly assigned patent applications U.S. Ser. No. 847,367, filed in the name of J. R. Holmes et al., and entitled "METHOD AND MEANS FOR REPLICATING CENTRALLY APERTURED VIDEO DISC RECORDS", and U.S. Ser. No. 031,205, filed in the name of J. R. Holmes, and entitled "MOLDING APPARATUS FOR PRODUCING CENTRALLY APERTURED RECORD DISCS". The machines described in these applications comprise injection molding machines having a molding assembly with first and second mold halves reciprocally movable between a closed position defining an annular mold cavity into which molten disc-forming material of plastic or the like is injected to form a video information disc, and an open position wherein the mold cavity parting line is opened and the resultant molded video information disc is removed from the machine. The mold cavity is lined by one or more disc-shaped stamping dies carried by the mold halves, and these stamping dies include surface geometries representative of the information to be formed on the disc.
In injection molding machines of this general type, it is well known to inject molten plastic material under pressure from an injector gun assembly through a hot sprue bushing and into the mold cavity. When the mold cavity is filled with the molten plastic material, the surrounding machine structure including the respective mold halves, the stamping dies, and the hot sprue bushing are commonly cooled by a circulating liquid coolant to reduce the temperature of the molten plastic material and thereby solidify the material. After solidification, the hardened information disc is removed from the machine and the cycle is restarted, thereby allowing a relatively rapid production rate of the information discs. However, the plastic material within the sprue bushing typically has a relatively large thickness compared with the plastic material within the mold cavity, whereby the plastic material within the sprue bushing takes the longest time to solidify. The cycle time for molding the information discs in machines of this type therefore tends to be dependent upon the longer cooling time of the material within the sprue bushing, and not upon the shorter cooling time of the material within the mold cavity. Moreover, this material within the sprue bushing when solidified comprises wasted plastic material which must be removed from the molded information disc, typically by means of a punch assembly or the like which simultaneously forms a central aperture in the disc to permit the disc to be placed upon the spindle of an appropriate playback machine.
Formation of the central aperture by means of the conventional punch assembly as disclosed generally in the above-referenced copending patent applications poses a variety of problems in the production of the information discs. For example, the punching step requires a certain cycle time to perform, thereby undesirably prolonging the cycle time for each information disc. Moreover, the use of the punch assembly results in an aperture shape having a concentricity dependent upon the state of wear of the punch die. As punch die wear progresses with deteriorating concentricity, the likelihood of the creation of debris in the vicinity of the aperture increases, and such debris can deleteriously affect the quality of a subsequently molded information disc. Finally, the use of a punch assembly places the information disc under substantial stress which can crack an otherwise acceptable disc in the region of the central aperture, resulting in rejection of the disc as scrap.
Some attempts have been proposed to mold the central aperture into the information disc prior to solidification of the disc-forming material and thereby avoid use of a punch assembly. See, for example, copending and commonly assigned U.S. application Ser. No. 202,824, filed in the name of A. Cane et al., and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE" which discloses a valve member movable across the mold cavity into engagement with the sprue bushing to mold the central aperture into the disc while controlling flow of molten material into the mold cavity. See also, for example, copending and commonly assigned U.S. application Ser. No. 202,838, filed in the name of B. F. P. Mayer, and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE" which discloses a poppet valve structure movable through the mold cavity and engageable with the sprue bushing to mold a central aperture into the disc while controlling flow of molten material into the mold cavity. By way of further example, see copending and commonly assigned U.S. application Ser. No. , filed in the name of A. Taylor, and entitled "HOT SPRUE VALVE ASSEMBLY FOR AN INJECTION MOLDING MACHINE" which discloses a sleeve-type valve cooperating with a sprue bushing to mold a central aperture into a record disc while controlling flow of molten material into the mold cavity. In all of these designs, however, relatively complex and expensive movable valve members and associated actuation devices are required to provide the controlled movement of the valve member with respect to the sprue bushing.
The present invention constitutes a substantial improvement over prior hot sprue valve assemblies by providing an improved hot sprue assembly wherein controlled fow of molten material into the mold cavity is achieved in the absence of movable valve members and associated actuation devices, and wherein a central aperture is molded directly into an information disc prior to solidification of disc-forming material.